What Is Cancer -- An Overview

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What is cancer?

Cancer is a word that refers to approximately 150 diseases that exhibit two characteristics in common: (1) an uncontrolled growth of cells and (2) the ability to invade and damage normal tissues either locally or at distant sites in the body. Ninety percent of human cancers arise in the epithelium (the layers of cells covering the body's surface and lining internal organs and various glands); these cancers are called carcinomas. Sarcomas are cancers of the supporting tissues of the body, such as bone, muscle and blood vessels. Cancers of the blood and the lymph glands are called leukemias and lymphomas, respectively. Gliomas are cancers of the nerve tissue. Melanomas arise from darkly pigmented cells, usually in the skin.

What causes cancer?

Cancer occurs when the DNA present in a gene is altered in such a way that the gene can no longer instruct the cell in which it resides to produce a certain protein in the normal manner. Such an alteration can take place when a gene is exposed to radiation or particular drugs or chemicals, or when some as yet unexplained internal signal occurs. These factors can cause the DNA within a gene to break and recombine incorrectly or to mutate. Once one of these changes has taken place, certain genes may be transformed into oncogenes, while other genes (cancer-suppressing genes called anti-oncogenes) may be inactivated.

If a gene has become an oncogene, the cell in which it is located may begin to produce unusually large amounts of one of its normal proteins or to manufacture an altered form of that protein. If an anti-oncogene has been rendered inactive, the cell containing it can no longer produce a normal protein whose function is to suppress cancer. In rare cases, an aberrant protein is manufactured when a cancer virus enters a cell and introduces an oncogene. Once any of these deviations in protein production has occurred, the cell alters its size, shape, surface characteristics and behavior. Thus, it becomes a cancer cell that is distinguishable from a normal cell.

How does cancer progress within the body?

Every cancer starts with a single cell that has been released from the growth restraints placed on all normal cells. Because the changes that have taken place within the cancer cell have been directed by the cell's DNA (the molecular basis of heredity), they are passed on to each of the daughter cells arising from the original cancer cell. Eventually, a family of abnormal cells is formed. Except in the case of leukemia, these cells form a mass, or tumor.

The cells of the tumor then push outward from their boundaries, infiltrating surrounding normal tissues. Small clumps of cells may then dislodge from the tumor and migrate to distant sites, often by invading the circulatory system of the blood or lymph. After traveling to a new organ, the cancer cells burrow out of the blood or lymph vessels and invade the surrounding tissues. There they continue to multiply, forming secondary tumors. This process of spreading to a distant site is called metastasis. Eventually, either local invasion or metastasis disrupts the body's normal functions and often leads to death.

Graphic Depiction -- The Oncogene: A Key Factor in the Development of Cancer Below
 

 

 

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The Oncogene: A Key Factor in the Development of Cancer



Under normal conditions, the DNA in a gene instructs the cell in which the gene resides to produce certain proteins in certain amounts.



If the cell is exposed to radiation or to certain drugs or chemicals, the DNA can break apart.



The separated DNA is then likely to recombine incorrectly, which may result in the formation of an oncogene.



Once the oncogene has been created, the cell may produce unusually large amounts of one of its normal proteins (or an aberrant protein). This causes the cell to transform into a cancer cell that looks very different from its former self .



When the cancer cell replicates its DNA and divides, each daughter cell possesses an identical oncogene. As this process of replication and division continues, cancer spreads.